I just took my Dash and AC apart to replace the evaporator, and I can confirm what they are saying.
I have seen posts about sunroof drains, and they clogging/leaking and it comes out behind interior panels by the footwells or behind the dash. Even windshield leaks that do the same, so its worth checking to eleminate that as a cause.
But, sloshing water sounds behind the glove box sounds like a clogged AC evaporator drain to me.
The heat/AC is in a box under the dash, just over the transmission hump in the floor. There plastic box actually has a tube formed in it and it fits over a hole in the floor with a seal, so no real tube attached to the box, the box itself has the tube formed in it at the bottom and just extends out past the floor of the vehicle.
The evaporator, heater core and air blend doors are all contained in the box over the transmission hump, but the blower motor and fan is on the extreme passenger side with a retangular box inbetween them to connect them and carry the air from the blower motor. So if water builds up in the evaporator/heater core section, it would slosh back and forth in the rectangular box connecting to the blower motor/fan.
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